This invention relates generally to microwave circuit components and, more particularly, to devices for providing phase modulation at microwave frequencies. An increasing number of microwave circuitry applications are implemented in the form of integrated circuitry, i.e. MMIC (microwave monolithic integrated circuitry). There is a requirement in many applications for a phase modulator that can be conveniently fabricated in MMIC form, for use in applications using frequency or phase modulation, such as in cellular telephones and other forms of microwave communication.
A well known technique for low-index phase modulation of microwave signals, i.e. modulation over a relatively small angular range, is to split an incoming signal into initially equal components (I and Q), and to subject one component to a 90-degree phase delay. If one of the components is then varied in amplitude in accordance with a modulation signal, this results in a variation in phase angle when the two components are recombined. One difficulty with this approach is that it produces amplitude modulation of the incoming signal. Moreover, the resultant phase shift is not linearly related to the amplitude of the modulating signal. Amplitude modulation of the output signal can be eliminated if the amplitudes of both quadrature signals are carefully controlled, but this is a relatively complex task.
The object of the invention is to provide a microwave phase-modulated output signal with only minimal accompanying amplitude modulation, and more linearly related to the modulating signal.